1 882.] Ichthyology in the years 1881-82. 771 



vesicle appears at the tail end of the embryo when the blastoderm 

 has rather more than half surrounded the vitellus, and this vesi- 

 cle is almost certainly the result of the invagination of the gas- 

 trula mouth or blastopore. From this vesicle, known as Kupffer's 

 vesicle, a canal proceeds forwards and opens on the dorsal face of 

 the embryo. The gastrula of teleostean fishes is thus the result 

 of an invagination at the tail, essentially as in Amphioxus, and is 

 not homologous with the gastrula of Haeckel. 



The pectoral fins originate from lateral folds, and their first 

 skeletal elements are a pair of cartilaginous rods which arc not 

 placed radially, but are concentric with the base of the fin. These 

 folds vary in their position, but are placed so far back that then- 

 genetic relation to the gill-arches appears improbable. The posi- 

 tion of the fin becomes more anterior with the x growth of the 

 embryo, and in the cod the base rotates through an angle of 

 nearly 90 to gain its upright position. ■ The shoulder girdle is 

 of mesoblastic origin. 



The median unpaired fins originate from a dorsal and ventral 

 natatory fold, which may be continuous, discontinuous from the 

 very first (Hippocampus), or discontinuous at an early stage. The 

 vent of the young fish appears Ion- before the mouth ; the intes- 

 tine develops from behind forward, ami it is probable the intestine 

 and medullary canal are primitively continuous by means of a 

 neurenteric canal. 



The investigations of Professor Ryder show wide differences 

 in the order and manner of development of the various organs ; 

 differences of a nature to show that embryology alone is a most 

 unsafe basis for classification. 



In the four-spined stickleback the cerebral vesicles are extra- 

 ordinarily large and the walls of the brain cavity very thin ; the 

 optic cups have a great space between the floor of the cup and 

 the lens; the pectoral folds originate unusually near to the gill- 

 metrical vitelline system of blood-channels. The corpuscles 

 appear to originate by budding off from knobbed cells of the 

 hypoblast of the venous sinus. 



The nest-constructing habits of the sg'cklebacks have long ago 

 been noticed, but from the observations of Mr. Seal and Professor 

 %der, it is now known that the male possesses a special spinning 



